As well known, a hybrid electric vehicle uses both an internal combustion engine and a battery power source. That is, the hybrid electric vehicle uses an efficient combination of power of the internal combustion engine and power of a motor.
Based on a power sharing ratio between the engine and the motor, the hybrid electric vehicle may be classified into a mild type hybrid electric vehicle and a hard type hybrid electric vehicle. The mild type hybrid electric vehicle (hereinafter, referred to as a mild hybrid electric vehicle) includes a mild hybrid starter & generator (MHSG), which starts the engine or generates electric power by using an output of the engine, instead of an alternator. The hard type hybrid electric vehicle is provided with an integrated starter & generator (ISG) which starts the engine or generates electric power by using the output of the engine, and a drive motor which drives the vehicle, and the ISG and the drive motor are separately provided.
In the case of the mild hybrid electric vehicle, there is no traveling mode in which the vehicle is driven only by torque of the MHSG, but torque of the engine may be supplemented by using the MHSG in accordance with a traveling state, and a battery (e.g., a 48V battery) may be charged by regenerative braking. Therefore, fuel economy of the mild hybrid electric vehicle may be improved.
As a battery which supplies electric power to the MHSG or is charged with electric power recovered by the MHSG, a lithium-ion battery is used. That is, in order to exhibit optimum performance of the MHSG, a battery, which is fast in charging and discharging speeds and has improved durability, is used.
However, the lithium-ion battery has a problem in that energy storage performance and charging and discharging efficiency deteriorate in an extremely low-temperature state. Therefore, at a below zero temperature, the lithium-ion battery cannot supply sufficient electric power to the MHSG, and as a result, the engine cannot be started.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention, and therefore, it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.